1. Field of the Invention
The present invention relates to the management of noise in a large-sized liquid crystal display device, particularly to a liquid crystal display device in which the generation of noise due to friction occurring between members of the liquid crystal display device caused by thermal expansion after activation thereof, such as the liquid crystal panel, diffusion plate and reinforcing plate for a backlight unit, and frame, is prevented.
2. Description of the Related Art
Recently manufactured liquid crystal display devices have become larger in size, where the diagonal dimension of the screen generally ranges from 20 inches to 30 inches. Likewise, liquid crystal display devices with screens even exceeding a diagonal dimension of 40 inches have been developed and used to produce large-sized television sets, video display devices, or display devices of workstations and CAD devices. Such large-sized liquid crystal display devices also require the quality of brightness of display devices which employ cathode ray tubes.
Since such liquid crystal display devices are usually placed on a desk or floor, or such similar stationary location, they do not have to be as thin as laptop personal computers, but must be of such appropriate thinness to fully exploit their characteristic features. Therefore, in many cases, a transmissive type of liquid crystal panel has been developed for which a direct type backlight structure has been adopted.
In the direct type backlight structure, several tubular light sources such as cold-cathode tubes are arranged in a parallel manner on the rear surface of the liquid crystal panel without using a light guiding plate, and serve to direct light to the liquid crystal panel. A reflector is arranged on the back surface of the backlight structure, while a diffusion plate is arranged on an output surface to diffuse the light emanating from the light sources, and thereby effect uniform light emission. The tubular light sources are held by a frame, in such manner that the frame mechanically affixes and supports both ends of the light sources to be electrically connected. Most of the power consumed by a liquid crystal display device comes from the backlight, which is also mainly responsible for the heat generated by the device. The quantity of power consumed and heat generated increases in relation to the number of tubular light sources employed for the backlight structure, which is associated with the enlargement of the liquid crystal display device.
Meanwhile, in the large-sized liquid crystal display devices, the backlight unit is made in the shape of a box in which a metal frame and a back surface reflector are usually combined with resin members. Optical members of the liquid crystal display device, such as the diffusion plate and the liquid crystal panel, which are made of various materials such as plastic and glass, are arranged in front of the backlight unit. However, the quality of materials differs, such that some are thin while some are thick in order to obtain necessary strength. Consequently, optical members of the liquid crystal display device, such as the diffusion plate and the supporting frame largely differ in thermal expansion coefficient and this poses a problem because when the liquid crystal display device is activated by turning the backlight source ON and heat is generated, the various members rub against each other and the frame and noise associated with the friction occurs. This problem posed by differences in thermal expansion causing friction between the frame and various members of the liquid crystal display device also occurs when the device is deactivated, and noise generated by the friction continues for a while. These points are described in detail with the aid of FIG. 9 showing an exploded perspective view of a conventional liquid crystal display device.
A liquid crystal display device 80 consists of a liquid crystal panel 83, a bezel cover 84, and a direct type backlight unit 82. In the backlight unit 82, a diffusion plate 87 is sandwiched and fixed between a picture-frame-shaped area 89 arranged on the top end of a frame 88 and a plurality of tubular light sources 85 supported by a light source supporting member 86. The back surface of the tubular light sources 85 is covered by a metal reflector 90.
The top end portion of the backlight unit 82, that is, the area upon which the liquid crystal panel 83 is placed is the picture-frame-shaped area 89 of the frame 88. The liquid crystal panel 83 is usually made of glass, and cannot be directly placed on the frame 88. Therefore, rubber or the like is adhered to that part of the area of the frame 88, with which the liquid crystal panel 83 comes into contact, and used as cushion.
Further, to align the liquid crystal panel 83 with the backlight unit 82, it is necessary to provide alignment portions to be applied to the end surface of the liquid crystal panel 83 on the top end portion thereof, and the alignment portions are separately provided by plastic. Friction occurs between the frame, the cushion, and the material applied and the liquid crystal panel due to differences in thermal expansion coefficient after the liquid crystal display device is activated, giving rise to creaks. Furthermore, in some of the conventional liquid crystal display devices, the diffusion plate is disposed so as to cover the frame opening of the backlight unit and the picture-frame-shaped frame for fixing the diffusion plate is laid thereon, or a plate-shaped frame is assembled and the diffusion plate is attached to the frame. In this case, creaks also occur when friction between the diffusion plate and the frame takes place as described above.
In the case of the large-sized liquid crystal display device in particular, the heat generated by the backlight after the device is activated is greater owing to the larger size of the liquid crystal panel and the diffusion plate, and therefore the differences in thermal expansion between various members and the frame are consequently enhanced, leading to a greater creak arising from the friction. After the liquid crystal display device is activated, the creak can last for about two hours and therefore, needs to be reduced.